1. Field of the Invention
The present invention relates to a program to make of cutting data for inner face of a dental prosthesis at a time of producing a dental prosthesis by cutting a block material using a CAD/CAM system.
2. Description of the Conventional Art
A dental prosthesis is produced by cutting a block material using a CAD/CAM system through the steps of taking an impression of the inside of an oral cavity (a teeth shape or a dentition shape) of a patient using a dental impression material, where the impression includes an abutment tooth of a portion for which the dental prosthesis is produced, producing a gypsum model based on the impression, and measuring three-dimensional coordinates information of dentition shapes using a laser measuring device, where the dentition shapes are a dentition shape on the side of a portion for which a dental prosthesis of the gypsum model is produced and a dentition shape on an opposite tooth side of the portion.
Then, a dental prosthesis, that is, a coping (an inner crown), a crown, or a bridge is designed based on the measured data. At this time, a designing operation to match a margin of a dental prosthesis with a margin line of an abutment tooth at which the dental prosthesis is fixed, a designing operation to keep a cement space at an inner face of a dental prosthesis, and a designing operation for the outer face of a dental prosthesis are carried out (for example, refer to Japanese Patent Application Laid Open No. 2002-224142, and FIG. 13 of Domestic Re-publication of PCT International Publication No. WO01/091664). When designing of a dental prosthesis is thus finished, a block material to be machined is selected, and an objective dental prosthesis is cut using an automatic cutting machine, and is subjected to trimming. Then, production of a dental prosthesis is completed. In such a dental CAD/CAM system, as a device to input three-dimensional coordinates information of a gypsum model including teeth to be treated, e.g., teeth needing a dental prosthesis, or the like, for example, a device to obtain three-dimensional shape data in a non-contact state by directly irradiating light to a gypsum model using an optical fiber and receiving the reflected light (for example, refer to Japanese Patent Application Laid Open No. 2002-224142), and a device to obtain three-dimensional shape data in a non-contact state by directly irradiating laser to a gypsum model (for example, refer to FIG. 13 of Domestic Re-publication of PCT International Publication No. WO01/091664) are used. Further, a device to obtain three-dimensional shape data by measuring the surface of a gypsum model by using a probe as a contact is also used (for example, refer to Japanese Patent Applications Laid Open No. 57-200144, 2002-336277, and 2003-61981).
Among the above-described measuring devices, an excellent three-dimensional measuring device which can lower a production cost and a maintenance cost because of having only one laser sensor to measure a shape of a gypsum model to be measured, is a three-dimensional measuring device having a rotary table in which an axis of a rotary shaft is along z axis, a mounting table fixedly provided on the rotary table to set a measured object mounting tool, and a measuring part to measure the three-dimensional coordinates (x, y, z) of the shape of a gypsum model to be measured, which is mounted on the measured object mounting tool on the mounting table, by one laser sensor capable of rotating and moving on the same plane including z axis around a desired point on the z axis at least. In such a dental CAD/CAM system, movement of a cutting bar of a general automatic cutting machine is limited to those on one plane with respect to a block for cutting and in the axial direction of the cutting bar, as illustrated in FIGS. 5 and 6 of Japanese Patent Application Laid Open No. 2003-61981. Thus, when the data for cutting an inner face of a dental prosthesis includes an under cut portion in the axial direction of a cutting bar, a portion which cannot be cut becomes to exist. Here, an under cut portion is a more recessed portion than a already cut portion when a cutting bar cut a certain portion and further advances to cut in a axial direction of the cutting bar. Further, in a dental CAD/CAM system, a program is generally made to stop cutting work when a portion which cannot be cut exists. Therefore, there is a problem that cutting may be not finished.
The reason of generating such a phenomenon is as follows. When data for cutting the inner face of a dental prosthesis is produced, the three-dimensional coordinates of a portion for which a dental prosthesis is produced are faithfully measured from a gypsum model and the coordinates are used as they are. Thus, when the longitudinal section of the three-dimensional coordinates-measured gypsum model includes a recessed part in a portion at the tooth root side than the occlusion face side of an abutment tooth, data for cutting the inner face of a dental prosthesis is produced while missing the existence of the recessed part. That is, when the data for cutting the inner face of a dental prosthesis is produced, the inner face except a portion of a margin line is automatically designed so as to keep a cement space having an approximately fixed thickness from the outer face of an abutment tooth. Thus, when the longitudinal section of the three-dimensional coordinates-measured gypsum model includes a recessed part in a portion at the tooth root side than the occlusion face side of an abutment tooth, the inner face is designed to have a projected part according to the shape of the recessed part.
Therefore, it is necessary to make data for cutting the inner face of a dental prosthesis while taking care so as not to form an under cut part in the axial direction of a cutting bar. However, it is hard in fact to carry out the operation to remove an under cut part while confirming three-dimensional data on a monitor, and much time is needed. Further, an under cut part is hardly displayed on a screen depending on a shape. Thus, data for cutting the inner face of a dental prosthesis is finally produced having an under cut part in the axial direction of a cutting bar, so that the actual cutting may be interrupted.